1. Field of the Invention
The invention is directed to discharge lamp ignition devices for starting high pressure discharge lamps used in projectors, particularly, mercury lamps, metal halide lamps, and xenon lamps, and projectors using such discharge lamp ignition devices.
2. Description of Related Art
High intensity discharge lamps (HID lamps) are used in projectors for optical equipment used for displaying graphic images, such as liquid crystal projectors and DLP® projectors. One method used in these projectors for displaying color images is to split the three colors—red (R), green (G), and blue (B)—using a dichromic prism or other means, generate three separate images with a space modulation element for each color, and then recombine the light paths using a dichromic prism or other means. Another method for displaying color images is to use a spin filter which is a dynamic color filter that comprises a color wheel that passes the three primary colors (R, G, B), sequentially to generate three colored luminous fluxes by passing light from the light source through this filter, and then, sequentially generate images in the three colors by time division by means of controlling a space modulation element in synchronization with the filter.
Among the discharge lamp ignition devices that start the discharge lamps described above, there are those which, with the voltage called the no-load discharge voltage impressed on the lamp at startup, impress a high voltage to generate dielectric breakdown within the discharge space to first bring about a glow discharge, then an arc discharge, and finally, a stable steady voltage. The lamp discharge voltage, which is a low value of perhaps 10 V at the time of transition to arc discharge, gradually increases as the temperature rises and stabilizes at a constant voltage in a state of constant lighting. Discharge lamp ignition devices normally have converters that match the output of the input power supply to the discharge voltage of the lamp so that the lamp current necessary to achieve the desired power of the lamp can be output; moreover, there is an arrangement to detect the lamp voltage or converter output voltage, and on the basis of that information, to determine the target lamp current from the quotient value of dividing the target voltage by the detected voltage.
As for methods of driving discharge lamps, there is the direct current drive method in which the lamp is lighted by the converter, and the alternating current drive method in which an inverter for cyclical reversal of polarity is provided at a stage subsequent to the converter. The direct current drive method has a great advantage in that the luminous flux from the lamp is of the direct current type and does not vary with time, and so it is basically possible to apply it in just the same way to both types of projectors described above. The alternating current drive method, on the other hand, has the advantage of using the freedom not found with the direct current drive method of polarity reversal frequency, and so it is possible to control the wear and service life of the discharge lamp electrodes, but there is also a disadvantage, as described below, that arises from the very existence of polarity reversal.
Normally, every reversal of polarity in alternating current drive causes a slight variation in lamp current or an extreme phenomenon, such as delayed change or overshoot, and this appears almost without moderation as a flicker in luminous flux from the lamp or a fluctuation, such as overshoot or vibration. Consequently, if it is applied to the projectors described above that use the time division method, there is the problem that the timing with which the images are produced in succession by time division will not match the timing of the polarity reversals of the lamp's alternating current drive and fluctuation of the display image will appear at the beat frequency; depending on the frequency of the beats, this can be very unsightly. It has been necessary, therefore, to devise some way to synchronize the timing of the inverter's reversal of polarity with the rotation of the color wheel, which has the drawback of complicating the discharge lamp ignition device.
In projectors using the DLP method, moreover, the brightness of each color of each pixel of the display image is controlled by the duty cycle of the individual pixel of the space modulation element. With the alternating current drive method, therefore, even if the timing is synchronized, if there is a long period of overshoot, vibration, or other fluctuation of the luminous flux when the polarity is reversed, it becomes necessary to devise either a way to not use the light during that period or a way to control the operation of each pixel of the space modulation element to suppress the fluctuation. The former course has the drawback of lowering the effective efficiency of the light beam, and the latter course has the drawback of greatly complicating the control of the space modulation element in the projector equipment.
Incidentally, the present inventors discovered one more problem that sometimes occurs in projectors using the time division method, whether of the direct current drive type or the alternating current drive type. That is, it has been found that the light beam's spectrum component that does not pass through the color wheel—that is, the spectrum component that is complementary to the light beam that passes through—will be reflected by the color wheel; depending on the arrangement of the lamp and the color wheel within the optical system, the light beam may be returned back through the optical system to the lamp and focused on the discharge plasma component or the electrode that faces it. This will generate the phenomenon of the lamp voltage being lowered or modulated by the returning light beam. Further, because the amount of modulation of the lamp voltage will vary with the spectrum of the returning light, there will be a cyclical fluctuation of the amount of modulation of the lamp voltage as the color wheel rotates.
The amount of modulation of lamp voltage is greatest when the transmitted color is red (R), followed by green (G) and blue (B) in that order. If the color wheel has a region that allows all colors—white light—to pass through, there will be very little returning light in that position and so there will be almost no modulation of the lamp voltage. For example, the effect was measured by constructing a DLP projector optical system with a high pressure mercury lamp in which the discharge space in a bulb made of quartz glass contained from 0.15 to 3.0 mg of mercury and was filled with bromine and argon gases, the electrodes were tungsten, the rated power was 200 W and the lamp voltage when burning steadily was 85 V. The result was that the amount of modulation was about 3% when the transmitted color was red and about 0% when the transmitted color was white.
The problem is that the fluctuation of lamp voltage due to the returning light does not correspond to the power actually introduced into the discharge plasma space, but to the apparent voltage between the electrodes measured from outside the lamp. The inter-electrode voltage measured from the outside has a component that depends on the power introduced into the discharge plasma space (which contributes to luminescence) and a cathode fall component that does not depend on the introduced power (which does not contribute to luminescence); the reason that the apparent modulation of lamp voltage is produced is thought to be that the phenomenon is relatively moderate and appears even when the fluctuating returning light only affects the former component, but when the effect extends to the latter component, because of the relatively rapid phenomenon of surface heating of the electrodes by irradiation with light, the state of electrode surface heating is manifest almost as it occurs.
Because the modulating of lamp voltage by the fluctuating returning light is apparent, as described above, if the target lamp current is determined purely on the basis of the lamp voltage, which is the converter output voltage, then the lamp voltage will not be a fixed value with the desired stability, but under the influence of the modulation of lamp voltage by the returning light beam it will have a waveform overlaid with a cyclical fluctuating component. For that reason, the luminous flux emitted from the lamp will similarly be overlaid with a cyclical fluctuating component, containing fluctuations arising from modulation of the lamp voltage by the returning light beam, which will have an adverse impact on the uniformity of the brightness of the images projected by the projector.
At this point, it may be thought that, because rotation of the color wheel is synchronized with control of the space modulation element, even though there is fluctuation of the luminous flux arising from modulation of the lamp voltage by the returning light beam, that fluctuation will also be synchronized with the rotation of the color wheel, and that because of synchronization with control of the space modulation element, the impact on the images projected by the projector will be negligible, appearing only as a slight infidelity of the display colors.
However, with discharge lamp ignition devices of the alternating current drive type, if there is a discrepancy in the timing of this fluctuation and the polarity reversal of the alternating current drive, then, an unsightly flicker at the beat frequency will appear in the image projected by the projector. Further, with either alternating current drive or direct current drive, unless the operation to detect the lamp voltage and determine the target lamp current is synchronized with the rotation of the color wheel, this fluctuation will become an unrecoverable jitter with respect to rotation of the color wheel and cause unrecoverable disruption of the results of modulation of pixel brightness by time division by the space modulation element; an unsightly flicker will appear in the images projected by the projector.
Consequently, it is possible that this flickering phenomenon caused by fluctuation of the luminous flux arising from modulation of the lamp voltage by the returning light beam can be avoided by means of both synchronization of the timing of the polarity reversal of the inverter with the rotation of the color wheel and synchronization of the operation to detect the lamp voltage and determine the target lamp current with the rotation of the color wheel.
However, there is always some wobble in the rotation of the color wheel—play in the rotating spindle—and the extent of the play always varies; this means that the constantly varying play will be included in the angle of the color wheel surface. Although this sort of play exists in the surface of the color wheel, the angle of the light beam passing through the color wheel in its fundamental application can be ignored as long as the surfaces of the color wheel can be deemed to be parallel and play in the position of the transmitted light beam can be ignored as long as neither the refractive index nor the thickness is extreme, and so costly measures to improve the wobble precision of the color wheel are not taken.
With regard to the light reflected from the color wheel, however, the angle of reflection of the reflected light beam will be amplified to double the angle of play of the rotating spindle, and so the amount of modulation of the lamp voltage by the returning light beam is not stable, and as a result the fluctuation of luminous flux emitted from the lamp is not stable either. This has an adverse influence on the uniformity of the brightness of the images projected by the projector, including the appearance of an unsightly flicker in the images projected by the projector. It is not possible to completely avoid this adverse influence, even by means of both synchronization of the timing of the polarity reversal of the inverter with the rotation of the color wheel and synchronization of the operation to detect the lamp voltage and determine the target lamp current with the rotation of the color wheel.
Accordingly, in order to assure the high quality of images projected by the projector, it is necessary to prevent fluctuation of the luminous flux arising from modulation of the lamp voltage by the returning light beam. It is understood that, for that reason, if the occurrence of a light beam returning from the dynamic color filter cannot be prevented optically, a means of avoiding the occurrence of this fluctuation must be devised in the mechanism for determining the target lamp current—for example, by devising something in connection with detection of the lamp voltage, which is the converter output voltage. The description above has spoken of the dynamic color filter as a rotating color wheel, but the situation is the same for other types of dynamic color filters, such as rotating color prisms. Techniques including means of detecting the lamp voltage or current have been proposed in the past to achieve stable power control.
For example, Japanese Pre-Grant Patent Publication H10-321388 (U.S. Pat. No. 6,333,607) describes an H-bridge circuit that converts power supplied by a direct current power supply to alternating current and supplies it to a discharge lamp and a means by which to supply the power supplied to the discharge lamp from the direct current power supply; excess voltage is impressed on the lamp at the time of polarity reversal and the lamp voltage is sampled except when the excess voltage is generated, and so there is a means to mask the lamp voltage during the period when the excess voltage is generated.
Also, Japanese Pre-Grant Patent Publication H11-283766 describes a proposal in which, in order to prevent current overshoot immediately after polarity reversal by the inverter and thus eliminate flickering, the power to a high pressure discharge lamp is fixed by providing a current limiting circuit that outputs the direct current voltage, an inverter that changes the direct current to alternating current, a control circuit that calculates the target current on the basis of the discharge lamp voltage, and a holding circuit that provides output by holding the discharge lamp current; the current limiting circuit is controlled so that the discharge lamp current that is output by the holding circuit is matched to the target current.
In addition, Pre-Grant Patent Publication 2004-296119 (U.S. Pat. No. 6,967,449) describes a proposal which, in order to control voltage and current overshoot during polarity reversal of alternating current rectangular waveform voltage and current, provides a converter that outputs power converted to direct current power by switching of the input power, an inverter that converts the direct current power supplied by the converter to alternating current rectangular waveform power and outputs it, and a control portion that includes a power calculation unit, a control target value setting unit, a correction signal generation unit, a converter control signal generation unit, and a pulsewidth control unit. The power calculation unit calculates the power from the voltage detection signal and the current detection signal detected on the converter output side and generates a power detection signal, and the control target value setting unit outputs an output power command value that controls the direct current power to a target value. The converter control signal generation unit is supplied the output power command value, the correction signal, and the power detection signal and outputs a signal corresponding to the error in the power detection signal relative to the output power command signal that has been corrected by the correction signal. The pulsewidth control unit provides pulses to the converter based on the signal from the converter control signal generation unit. Within such an arrangement, the correction signal generation unit generates a correction signal that corrects the output power command value in response to the power detection signal, and thus outputs the correction signal in synchronization with the polarity reversal of the alternating current rectangular waveform power.
Japanese Pre-Grant Patent Publication 2005-071630 describes illuminating equipment that has a controllable direct current power supply, a full-bridge inverter that converts the direct current voltage from the controllable direct current power supply to alternating current voltage, and a means of control that controls the output of the controllable direct current power supply. In order to control the direct current voltage output of the controllable direct current power supply with high precision and enable service life judgments, the means of control samples one or both of the lamp voltage and lamp current at the time of lamp ignition, including the excess portion during polarity reversal, and forms a control signal that it supplies to the controllable direct current power supply.
However, the problem of fluctuation of luminous flux arising from modulation of the lamp voltage by the returning light beam arises from disruption by excessive modulation of the very lamp voltage that is necessary in order to decide the target lamp current, whether or not there are excessive electrical phenomena such as overshoot during polarity reversal, and it has been impossible to solve that problem with existing technology.